How are you taking flats? sky flats or LED panel? I bet it's the LED panel because if so then there's a known issue to these horizontal bandings but the solution is quite simple. Are you using the dimmer with your panel? What ADU value are you aiming for? Are you diffusing the light source somehow?
LEG panels work on the basis of on and off mechanism, when you have the LED panel dimmed down, camera is so sensitive that it will pick up this variation so the solution is as below.
Crank up the light source, put something like a few pieces of white paper in between your panel and scope so that you are able to diffuse the light and aim for around 22k ADU.
Hope this helps.
